A temperature compensated crystal oscillator (TCXO: Temperature Compensated Xtal Oscillator) is used in various electronic devices, such as a communication device and an information device.
FIG. 1 is a cross-sectional view of a structure of a conventional temperature compensated crystal oscillator. The conventional temperature compensated crystal oscillator includes a crystal oscillator 2 and an IC (Integrated Circuit) 3 that are disposed inside a casing 1. As illustrated in FIG. 1, the crystal oscillator 2 includes a crystal piece 21 and two (a pair of) excitation electrodes 22, 23 that are connected to the crystal piece 21.
The casing 1 has a lid 1A, and is made of ceramics. The casing 1 is hermetically sealed and filled with dry nitrogen, with the crystal oscillator 2 and the IC 3 disposed inside. The casing 1 is disposed on a circuit board 5.
The crystal piece 21 of the crystal oscillator 2 is connected to an inner wall 1B of the casing 1, such that the crystal piece 21 is located at a substantially center position of the internal space of the casing 1. The crystal piece 21 is an AT-cut quartz crystal having a particular thickness for attaining a target unique oscillation frequency. The excitation electrodes 22, 23 are formed on the crystal piece 21. The excitation electrodes 22, 23 are film electrodes made of gold (Au).
The IC 3 is disposed at the bottom of the internal space of the casing 1.
A temperature sensor 4 is disposed inside or on top of the IC 3. The temperature sensor 4 is a thermosensor whose resistance value changes according to the temperature of the IC 3. For example, a nichrome wire may be used as the temperature sensor 4. The resistance value of the temperature sensor 4 changes according to the temperature of the IC 3, and therefore the temperature sensor 4 outputs an electric current corresponding to the temperature of the IC 3.
FIG. 2 illustrates a circuit of the conventional temperature compensated crystal oscillator illustrated in FIG. 1.
The IC 3 includes a variable capacitor 31, an inverter 32, an output buffer circuit 33, a correction circuit 34, and a memory 35.
The variable capacitor 31 and the inverter 32 are connected to the excitation electrodes 22, 23 of the crystal oscillator 2, thereby forming a loop-type oscillation circuit including the crystal oscillator 2.
The output buffer circuit 33 converts oscillation signals obtained by the oscillation circuit into clock signals, and outputs the clock signals. In practical situations, the output buffer circuit 33 may have plural inverters; however, as a matter of convenience, only one inverter is illustrated in FIG. 2.
The variable capacitor 31 is a variable capacitance element whose electrostatic capacitance is variable. The variable capacitor 31 is inserted in series in the oscillation circuit, so that the electrostatic capacitance of the loop-type oscillation circuit may be varied. The variable capacitor 31 is formed with a variable diode such as a varicap diode. The electrostatic capacitance of the variable capacitor 31 may be varied according to the voltage applied from the correction circuit 34.
The memory 35 is built in the IC 3, and stores data expressing inverse properties of the frequency temperature properties of the crystal oscillator 2. The memory 35 is used by the correction circuit 34 for converting the current values expressing temperature signals into voltage values applied to the variable capacitor 31. The correction circuit 34 refers to the memory 35, and applies, to the variable capacitor 31, a voltage corresponding to the temperature signal (current value) expressing the temperature detected by the temperature sensor 4. For example, the correction circuit 34 has a circuit configuration as described in Japanese Laid-Open Patent Application No. 2008-300978 (see FIG. 3).
According to the configuration described above, when the temperature detected by the temperature sensor 4 changes, the electrostatic capacitance of the variable capacitor 31 is adjusted. Therefore, the oscillation frequency is stabilized with respect to temperature changes.
Clock signals output from the output buffer circuit 33 of the above-described temperature compensated crystal oscillator are used in a CPU (Central Processing Unit) or a communications unit of an electronic device.
As electronic devices are becoming miniaturized, electronic devices including temperature compensated crystal oscillators are becoming increasingly densified. For example, limited space is available in mobile phones and car navigation systems, and therefore such electronic devices are highly densified.
In manufacturing such a highly-densified electronic device, there is limited freedom in designing the internal structure of the electronic device.
An electronic device is typically provided with an electronic component that functions as a high-temperature heat source, such as the transmission amplifier of a mobile phone. This electronic component is mounted on a printed-circuit board together with a temperature compensated crystal oscillator. Thus, it is difficult to change the arrangement of electronic components in an attempt to reduce the amount of heat transferred from the heat source to the temperature compensated crystal oscillator.
Although the freedom in designing electronic devices is limited, electronic devices are becoming increasingly high-performance, and therefore there is growing demand for high-precision temperature compensated crystal oscillators.
Accordingly, there is growing demand for temperature compensated crystal oscillators capable of compensating the oscillation frequency with high precision, even in an environment with variable temperature.
However, in the above-described conventional temperature compensated crystal oscillator, the temperature sensor 4 is attached to the IC 3 that is spaced away from the crystal piece 21. The IC 3 is closer to a heat source mounted on the printed-circuit board, than is the crystal piece 21. Hence, there are cases where the temperature measured by the temperature sensor 4 is different from the actual temperature of the crystal piece 21, due to a delay in the heat transfer. Such a temperature difference caused by the delay in the heat transfer tends to increase when the temperature of the heat source rises rapidly.
Furthermore, the temperature difference attributed to the delay in the heat transfer may cause an error in the operation of compensating for the variation in the oscillation frequency. Accordingly, the variation in the oscillation frequency may not be appropriately compensated.
Another example of the conventional technology is a discrete type temperature compensated crystal oscillator, in which the correction circuit or the memory is provided separately from the IC. In this case also, the temperature sensor is spaced away from the crystal oscillator, and therefore a temperature difference is caused by the delay in the heat transfer, similar to the case of the temperature compensated crystal oscillator in which the correction circuit and the memory are provided together in the IC. Accordingly, the variation in the oscillation frequency may not be appropriately compensated.
In order to reduce the errors in the control operation caused by the delay in the heat transfer, a temperature sensor such as a nichrome wire may be directly attached to the crystal oscillator. However, the mass and the position of the excitation electrodes of the crystal oscillator are determined for attaining a target oscillation frequency. Thus, it is difficult to directly attach a temperature sensor such as nichrome wire to the crystal oscillator.